This invention relates to a photosensitive member and, more particularly, to a photosensitive member in electrophotography.
Since the invention of Carlson's method (U.S. Pat. No. 222,176, 1938), electrophotography has been making remarkable progress in applicability and commercial introduction and there have since been various materials developed and introduced as photosensitive members in electrophotography.
The photosensitive materials which have found use mainly in electrophotography are: in the area of inorganic substances, amorphous selenium, arsenic selenide, tellurium selenide, cadmium sulfide, zinc oxide, amorphous silicon, etc., and in the area of organic substances, polyvinyl carbazole, metallic phthalocyanine, disazo pigments, trisazo pigments, perylene pigments, triphenylmethane compounds, triphenylamine compounds, hydrazone compounds, styryl compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, etc.
These photosensitive materials have constituted the required photosensitive members, some forming monolayers of simple substances, some dispersed in some binding agent forming dispersions in binders, and others in the form of laminates, each functionally composed of a charge generating layer and a charge transporting layer.
Such photosensitive materials, however, have exhibited defects when used in electrophotography in the past.
One of the defects has been a harmfulness to human health: with the exception of amorphous silicon, all the inorganic substances referred to above have properties detrimental to human health.
On the other hand, a photosensitive member in practical use in a copying machine is required always to be stable in properties to rigorous conditions and environmental problems, such as those concerning electrostatic charging, exposure to light, development, transferring, static elimination, and cleaning. In this respect, all the organic substances enumerated above are lacking in durability and, when used, instability has come to the fore in many points of the useful properties.
As a means to solve the above-mentioned problems, amorphous silicon (hereinafter abbreviated to "a-Si"), made by the plasma chemical vapor deposition process (hereinafter called "plasma CVD process"), has in recent years been finding application as a photosensitive material, especially in electrophotography.
The photosensitive material a-Si has various splendid properties. However, its use raises a problem that, because of a large specific inductive capacity epsilon approximately 12, a-Si essentially needs to form a film with a minimum thickness of approximately 25 microns in order for the photosensitive member to have a sufficient surface potential.
The production of a-Si photosensitive members by the plasma CVD process is a time-consuming operation with the a-Sifilm formed at a slow rate of deposition, and, moreover, the more difficult it becomes to obtain s-Si films of uniform quality, the longer it takes for the films to be formed. Consequently, there is a high probability that an a-Si photosensitive member in the use causes defects in images, such as white spot noise, besides other defects including an increase in cost of the raw material.
In any attempt for improvement that has been made concerning the above-mentioned defects, it was essentially undesirable to make the film thickness smaller than the minimum mentioned above.
Furthermore, the a-Si photosensitive material exhibits defects in adhesivity to the substrate, in corona resistance and resistance to environment and also chemicals.
As an answer to the problems described above, it has been proposed to provide an a-Si photosensitive layer with an overcoating layer or an undercoating layer of an organic plasmapolymerized film: examples describing the overcoating were announced in Japanese Patent KOKAI Nos. 61761/1985, 214859/1984, 46130/1976, U.S. Pat. No. 3,956,525, etc. and those describing the undercoating in Japanese Patent KOKAI Nos. 63541/1985, 136742/1984, 38753/1984, 28161/1984, 60447/1981, etc.
It is known that an organic plasma-polymerized film can be made from any of gaseous organic compounds, such as ethylene gas, benzene and aromatic silane, (one reference in this respect is the Journal of Applied Polymer Science, 1973, 17 (885-892) contributed by A.T. Bell, M. Shen et al.), but any such organic plasma-polymerized film produced by a conventional method has been in use only where its insulation property is required to be good. Films of this kind have been regarded as insulators having electrical resistance of approximately 10.sup.16 ohm cm, such as an ordinary polyethylene film, or at the least as materials practically similar to an insulator in the application.
The Japanese Patent KOKAI No. 61761/1985 made public a photosensitive member coated with a surface protective layer which is a carbon insulation film resembling diamond with a film thickness of 500 angstrom-2 microns. This thin carbon film is designed to improve a-Si photosensitive members with respect to their resistance to corona discharge and mechanical strength. The polymer film is very thin and an electric charge passes within the film by a tunnel effect, the film itself not needing an ability to transport an electric charge. The publication lacked a description relating to the carrier-transporting property of the organic plasma-polymerized film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 214859/1984 made public the use of an overcoating layer of an organic transparent film with thickness of approximately 5 microns which can be made from an organic hydrocarbon monomer, such as ethylene and acetylene, by a technique of plasma polymerization. The layer described therein was designed to improve a-Si photosensitive members with respect to separation of the film from the substrate, durability, pinholes, and production efficiency. The publication lacked a description relating to the carrier-transporting property of the organic plasma-polymerized film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 46130/1976 made public a photosensitive member utilizing n-vinylcarbazole, wherein an organic plasma-polymerized film with thickness of 3 microns-0.001 microns was formed at the surface by a technique of glow discharge. The purpose of this technique was to make bipolar charging applicable to a photosensitive member of poly-n-vinylcarbazole, to which otherwise only positive charging had been applicable. The plasma-polymerized film is produced in a very thin layer of 0.001 microns--3 microns and used by way of overcoating. The polymer layer is very thin, and it is not considered necessary for it to have an ability for the transportation of an electric charge. The publication lacked a description relating to the carrier transporting property of the polymer layer and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The U.S. Pat. No. 3,956,525 made public a technique whereby on a substrate a layer of a sensitizer is laid and thereupon a layer of an organic photoconductive electric insulator is superimposed and the laminate is overlaid by a polymer film 0.1 micron -1 micron thick formed by a technique of glow discharge. This film is designed to protect the surface so as to make the photosensitive members resistant to wet developing and therefore used by way of overcoating. The polymer film is very thin and does not need an ability to transport an electric charge. The publication lacked a description relating to the carrier transporting property of the polymer film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 63541/1985 made public a photosensitive member wherein an a-Si layer is undercoated by an organic plasma-polymerized film resembling diamond with a thickness of 200 angstrom 2 microns. The organic plasma-polymerized film is designed to improve the adhesivity of the a-Si layer to the substrate. The polymer film can be made very thin and an electric charge passes within the film by a tunnel effect, the film itself not needing an ability to transport an electric charge. The publication lacked a description relating to the carrier transporting property of the organic plasma-polymerized film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 28161/1984 made public a photosensitive member wherein on a substrate an a-Si film is laid and thereupon an organic plasma-polymerized film is superimposed. The organic plasma-polymerized film is used as an undercoat, the insulation property thereby being utilized, and also has the functions of blocking, improving the adhesivity, or preventing the separation of the photosensitive coat. The polymer film can be made very thin and an electric charge passes within the film by a tunnel effect, the film itself not needing an ability to transport an electric charge. The publication lacked a description relating to the carrier transporting property of the organic plasma polymerized film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 38753/1984 made public a technique whereby an organic plasma polymerized thin film with a thickness of 10-100 angstrom is formed from a mixed gas composed of oxygen, nitrogen and a hydrocarbon, by a technique of plasma polymerization and thereupon an a-Si layer is formed. Said organic plasma-polymerized film is used as an undercoat utilizing the insulation property of the polymer and also has the functions of blocking or preventing the separation of the photosensitive coat. The polymer film can be made very thin and an electric charge passes within the film by a tunnel effect, the film itself not needing an ability to transport an electric charge. The publication lacked a description relating to the carrier transporting property of the organic plasma-polymerized film and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 136742/1984 described a semiconductor device wherein on a substrate an organic plasma-polymerized layer with thickness of approximately 5 microns was formed and thereon a silicon layer was superimposed. Said organic plasma-polymerized layer was designed to prevent the aluminum, the material forming the substrate, from diffusing into the a-Si, but the publication lacked description relating to the method of its fabrication, its quality, etc. The publication also lacked a description relating to the carrier transporting property of the organic plasma-polymerized layer and the topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.
The Japanese Patent KOKAI No. 60447/1981 made public a method of forming an organic photoconductive layer by plasma polymerization. The publication lacked description relating to the applicability of the invention to electrophotography. The description in the publication dealt with said layer as a charge generating layer or a photoconductive layer and the invention described thereby differs from the present invention. The topic matter failed to provide a solution to the essential problems of a-Si in the foregoing description.